Many crystal growth methods wherein a previously prepared fine AgX emulsion grain is fed to a reaction vessel containing seed crystals of an AgX emulsion with stirring, and the fine grains are dissolved in the reaction vessel by Ostwald ripening to grow seed crystals have been proposed. For example, such methods are described in U.S. Pat. No. 4,242,445, JP-A-55-142329 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-63-151618, JP-A-58-113926 to JP-A-58-113928, JP-A-57-23932, U.S. Pat. Nos. 2,146,938 and 3,317,322 and U.S. Pat. No. 4,879,208. In a system wherein an aqueous solution of a silver salt and an aqueous solution of a halide salt (hereinafter referred to as an X.sup.- salt) are added, a distribution of each their concentrations is present in the reaction solution until they are added and uniformly mixed. That is, the ununiformity of concentration of the silver salt and the halide salt occurs in the reaction solution. The distribution of the concentrations is continuously produced during the course of the addition thereof. It is believed that the reproducibility of the growth of AgX grains, the uniformity of the halogen composition, the uniformity of the formation of reduced silver nuclei, etc. are affected by the distribution of the concentrations. On the other hand, the above-described fine grain addition method has the advantage that fine grains are gradually dissolved to thereby supply solute ions so that the method is freed from the above-described disadvantages.
However, this method still has the following problems to be solved.
(1) There is the problem that the surface Gibbs free energy of the previously prepared fine grains is generally high and hence the fine grains are unstable and tend to be changed during storage. JP-A-57-23932 discloses a method wherein a fine grain emulsion prepared in the presence of a growth inhibitor is washed with water, dispersed and further re-dissolved and addition is then made. Since a growth inhibitor is present, the change in grain size during the storage of the emulsion is reduced, but there is the disadvantage that when the emulsion is added, the fine grains dissolve with difficultly.
(2) U.S. Pat. No. 4,879,208 discloses that in order to solve the above-described problem, a mixer is provided outside the reactor, an aqueous solution of a silver salt, an aqueous solution of an X.sup.- salt and an aqueous solution of a protective colloid are continuously fed to the mixer using a triple jet process and mixed to form fine AgX grains and the fine grain emulsion is continuously fed to the reaction vessel. In this method, the fine grains are fed to the reactor immediately after the formation thereof and the problem with regard to preservability does not occur. However, since an aqueous solution containing a silver salt at a high concentration and an aqueous solution containing an X.sup.- salt at a high concentration are directly mixed with each other, there is the disadvantage that degree of the supersaturation in the mixing zone becomes very high and the proportion (by number) of multiple twin grains in the fine grains becomes high (namely more than 2%). Further, the grain size distribution is broadened. When such fine grains are added, a problem occurs in that multiple twinned grains in the fine grains are grown and new nuclei tend to be formed.
(3) When the flow rate of the aqueous solution of the protective colloid is increased, or the aqueous solution of the silver salt or the aqueous solution of the X.sup.- salt is diluted to prevent the above-described problem from occurring, there is the disadvantage that the total amount of the solution to be added is greatly increased. Further, there is the disadvantage that because the method is a continuous process, there is the difficulty in increasing the amount of silver/ml. Accordingly, this method gives rise to problems in that the total amount of the solutions to be added is increased, the amount by mol of AgX prepared in a given reactor is reduced and production efficiency can not be increased.
(4) When the aqueous solution of the silver salt, the aqueous solution of the X.sup.- salt and the aqueous solution of the protective colloid are added according to the continuous method described in U.S. Pat. No. 4,879,208, they are not well-reacted in the mixer and there is the possibility that the soluble salts as such are added to the reactor. Moreover, there is the disadvantage that some fine grains stay in the mixer a long period of time and other fine grains stay only a short period of time. Thus, the residence time of the fine grains in the mixer is not uniform and hence, control can not be sufficiently made.
(5) With regard to the characteristics of fine grains to be added in the above-described fine grain addition method, fine grains having a grain size (i.e., diameter) of not larger than 0.1 .mu.m are merely specified. Other characteristics (e.g., population of multiple-twinned grains) are not set forth at all. Moreover, there is no description that the supersaturation degree during the growth of the seed crystals is controlled depending on the grain size specified.
U.S. Pat. No. 4,242,445, JP-A-55-142329, JP-A-63-151618, JP-A-58-113926, JP-A-58-113927, JP-A-58-113928 and JP-A-57-23932 disclose the addition of a fine grain emulsion comprising fine grains having a grain size (i.e., diameter) of not larger than 0.1 .mu.m, but do not disclose a method for preparing the fine grains and characteristics of the fine grains.
U.S. Pat. No. 2,146,938 discloses coarse grains formed by ripening the fine grain emulsion (i.e., by the Ostwald ripening between fine grains), which is different from the present invention. That is, the fine grain emulsion washed is disclosed in U.S. Pat. No. 2,144,938.
U.S. Pat. No. 3,317,322 discloses the formation of shell due to mixing a fine grain emulsion which is not subjected to chemical sensitization in a core emulsion which is subjected to chemical sensitization, but does not disclose a detail method for preparing the fine grain emulsion. That is, when according to Trivelli and Smith, The Photographic Journal, Vol. LXXIX, (May, 1939), P.P. 330-338, the proportion (by number) of multiple-twinned grains is measured, the value is 5% or more. Also, in U.S. Pat. No. 3,317,322, the prevention of mixing the multiple-twinned grains is not considered.